The present invention relates to a method of manufacturing an article by diffusion bonding and superplastic forming.
It is known to manufacture hollow metallic articles by diffusion bonding and superplastic forming metal workpieces. These metal workpieces include elementary metal, metal alloys, intermetallic materials and metal matrix composites.
The diffusion bonding and superplastic forming process may be used to produce contoured articles for example fan blades, or fan duct outlet guide vanes, for gas turbine engines by superplastically, or hot forming, an integral structure formed by the diffusion bonding process.
A procedure for manufacturing an article by diffusion bonding and superplastic forming is disclosed in our European patent EP0568201B. In EP0568201B the integral structure formed by the diffusion bonding process is twisted before the integral structure is superplastically formed. Additionally the integral structure is hot creep formed in the superplastic forming dies.
Additionally our UK patent GB2306353B discloses manufacturing a fan blade by diffusion bonding and superplastic forming. In GB2306353B the integral structure is formed from two metallic workpieces which subsequently define the outer profile of the fan blade. The two metallic workpieces are produced by cutting an inclined slot through a parallelepiped metal block to produce two longitudinally tapering metallic workpieces. The thicker ends of the metallic workpieces are aligned to form the root of the fan blade and the remainder of the metallic workpieces are machined to the appropriate thickness to give the required mass distribution.
This manufacturing process requires that the thickness of the original parallelepiped metallic block is about half, just less than half, of the thickness of the root of the finished fan blade in order to allow machining to produce the root. A problem with this process is that it is wasteful of metal, machining time and is expensive. Additionally the microstructure of the parallelepiped metallic block is not the optimum microstructure, due to the thickness of the original metallic block.
The problem is partially overcome, as also disclosed in GB2306353B, by using thinner parallelepiped metallic blocks and adding extra small blocks at the thicker ends of the two longitudinally tapering metallic workpieces to form the root of the fan blade. However, this process is still wasteful of metal, machining time and is expensive. The microstructure of the parallelepiped block is improved due to the smaller thickness of the parallelepiped block. But there are the additional requirements of welding on the extra small blocks and evacuating the spaces between the metallic workpieces and the blocks to ensure a diffusion bond forms. The microstructure of the metallic workpieces is still not the optimum microstructure due to the thickness of the original parallelepiped metallic block.
Accordingly the present invention seeks to provide a novel method of manufacturing an article by diffusion bonding which overcomes the above mentioned problems.
Accordingly the present invention provides a method of manufacturing an article of predetermined finished profile by diffusion bonding and superplastic forming at least two metal workpieces comprising the steps of:
(a) forming at least two metal workpieces, each metal workpiece has at least one surface,
(b) applying a stop off material to prevent diffusion bonding to preselected areas of at least one of the surfaces of at least one of the at least two metal workpieces,
(c) assembling the at least two metal workpieces into a stack relative to each other so that the surfaces are in mating abutment,
(d) applying heat and pressure across the thickness of the at least two metal workpieces to diffusion bond the at least two metal workpieces together in areas other than the preselected areas to form an integral structure,
(e) forging one end of the integral structure to produce a region of increased thickness,
(f) heating the integral structure and internally pressurising the integral structure to cause the preselected area of at least one of the metal workpieces to be hot formed to produce a hollow article of predetermined shape.
Preferably the method comprises after step (e) and before step (f) placing the integral structure in a hot creep forming die, heating the integral structure while it is within the die to cause the integral structure to be hot creep formed on the convex surface of the die.
Preferably step (a) comprises forming a first metallic workpiece, a second metallic workpiece and a third metallic workpiece, the third metallic workpiece having two flat surfaces, and step (c) comprises assembling the three metal workpieces into the stack relative to each other so that third metal workpiece is between the first and second metal workpieces and the flat surfaces are in mating abutment.
The hollow article may be a fan blade or a compressor blade.
Alternatively step (a) comprises forming a first metallic workpiece and a second metallic workpiece.
The hollow article may be a fan outlet guide vane, a compressor blade or a fan blade.
After diffusion bonding the stack of workpieces and before superplastically forming the integral structure, the integral structure may be heated and loads may be applied to opposite ends of the integral structure to twist one end relative to the other end to contour the integral structure to a predetermined shape.
After twisting the integral structure and before superplastic forming the integral structure, the contoured integral structure may be internally pressurised to break the adhesive bond between the stop off material and the at least one workpiece in the preselected area.
Preferably after internally pressurising the integral structure to break the adhesive bond and before internally pressurising the integral structure to superplastically form at least one metal workpiece, the interior of the integral structure is sequentially evacuated and supplied with inert gas to remove oxygen from the interior of the integral structure.
Preferably after diffusion bonding the stack of workpieces and before superplastically forming the integral structure, the integral structure is internally pressurised to break the adhesive bond between the stop off material and the at least one workpiece in the preselected area.
Preferably after the metal workpieces are arranged in a stack and before the metal workpieces are diffusion bonded together to form an integral structure, the edges of the metal workpieces are sealed.
Preferably the edges of the metal workpieces are welded together.
Preferably where the metal workpieces are made of a titanium alloy, the metal workpieces are heated to a temperature equal to, or greater than, 850xc2x0 C. and the pressure applied is equal to, or greater than, 20xc3x97105 Nmxe2x88x922 to diffusion bond the workpieces together to form an integral structure.
Preferably the metal workpieces are heated to a temperature between 900xc2x0 C. and 950xc2x0 C. and the pressure applied is between 20xc3x97105 Nmxe2x88x922 and 30xc3x97105 Nmxe2x88x922.
Preferably the integral structure is heated to a temperature equal to, or greater than, 850xc2x0 C. to superplastically form the integral structure.
Preferably the integral structure is heated to a temperature between 900xc2x0 and 950xc2x0 C.
Preferably the integral structure is hot creep formed at a temperature equal to, or greater than, 740xc2x0 C.
Preferably step (e) comprises upset forging.
Preferably the region of increased thickness is machined. Preferably the region of increased thickness is subsequently machined to form a dovetail root or a firtree root. Preferably step (e) comprises heating the integral structure to a predetermined temperature before forging.
Preferably the integral structure is heated to a temperature between 900xc2x0 C. and 950xc2x0 C.
Preferably in step (a) each of the at least two metal workpieces has at least one flat surface.
Preferably step (a) comprises forming at least one longitudinally tapering metal workpiece and step (e) comprises upset forging the thicker end of the metal workpiece.
Preferably step (a) comprises forming two longitudinally tapering metal workpieces, step (c) comprises arranging the thicker ends of the metal workpieces at one end of the stack and step (e) comprises upset forging the thicker ends of the metal workpieces.